This invention relates to a novel method of electrically processing a completely-assembled CRT (cathode-ray tube) to reduce afterglow during the subsequent operation of the CRT.
A CRT comprises an evacuated envelope which includes a neck, a funnel and a faceplate opposite the neck. A luminescent viewing screen is supported on the internal surface of the faceplate. A conductive coating on the inside of the funnel is one plate of a filter capacitor, and also is the anode of the CRT. An external coating on the funnel is the other plate of the filter capacitor. A mount assembly supported from a glass stem and including one or more electron guns is sealed into the neck. Each electron gun includes a cathode, a control electrode, a screen electrode, a focus electrode and a high-voltage electrode.
After the CRT is completely assembled and evacuated, the mount assembly is electrically processed so that the electron gun or guns become operative, their operation stabilized and their operating lives lengthened. This processing includes, in succession, (i) "hot-shot" wherein the cathodes are rendered electron-emitting, (ii) "low-voltage aging" wherein the electron emissions are stabilized and (iii) "spot-knocking" wherein spurious electron emission from the electrodes is reduced and the operation of the CRT is further stabilized.
Spot-knocking may be conducted before the hot-shot and also after the IIP (integral implosion protection) structure is mounted on the CRT. In ordinary spot-knocking, which is usually conducted before the hot-shot, all of the gun elements not connected to the anode are connected to ground potential, and positive low-frequency pulses are applied to the anode, causing spontaneous arcing to occur between gun elements connected to the anode and the adjacent gun elements. RF spot-knocking, which is usually conducted after the IIP structure is mounted on the CRT, is similar to ordinary spot-knocking except that RF (radio frequency) pulses are superimposed upon the low-frequency pulses, causing stimulated or forced arcing to occur between the gun elements connected to the anode and the adjacent gun elements.
A completed CRT, installed in a chassis, and operated in a normal manner, may continue to emit light from the viewing screen after the normal operating voltages are removed from the mount assembly. This effect, which may linger for minutes or hours, is referred to as afterglow and is attributed to the coincidence of two factors. First, a large residual electrostatic charge remains on the filter capacitor (which is integral with the CRT) after the operating voltages are removed. Therefore, a residual high voltage remains on the anode of the CRT and on the high-voltage electrodes of the mount assembly, which are connected to the anode, with respect to the other electrodes of the mount assembly. Second, there are sites on the electrodes of the electron gun from which electrons can be emitted when they are under the influence of the electric field produced by the residual high voltage on the high-voltage electrodes. Emitted electrons under the influence of the electric field move toward, and impinge upon, the viewing screen producing the afterglow.